I'm looking for an SMS gateway that has the following criteria:
- Supports sending from short codes
- Opt-in capabilities for end-user (will require user to opt in before sending message)
- Has a clean API, preferably an XML/ REST API, although SOAP is fine
- The cheaper the better
Honestly, I keep searching and searching all permutations of "SMS Gateway API" etc... on Google and I can't find any great services that can support all of these criteria. There are a lot of really SEO optimized sites and I can't tell which of them are legit. I'm looking for something that will be easy and quick to implement, and none of my colleagues have any recommendations. What APIs have you used (preferably with PHP) that have gotten this job done? Which services have been the most pleasant to deal with, both technically and support-wise?
Update: Will be sending primarily (probably solely) in the US.